Strain-induced Quantum Dots Research Articles

Magnetoluminescence from strain-induced quantum dots

We have calculated the effects of valence-band coupling on the magnetoluminescence spectra from an ${\mathrm{In}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As/GaAs quantum-well dot induced by an InP stressor island. The theory is based on the single-particle Luttinger-Kohn Hamiltonian using a realistic confinement potential. The band coupling strongly influences the calculated spectra. The numerical results were compared with experiments and a good agreement was obtained. Numerical results predict a reversal of the mesoscopic Zeeman splitting for the ground state and an avoided crossing of excited states.

MOCVD growth and optical characterization of strain-induced quantum dots with InP island stressors

Coherent InP islands, grown by low pressure metal organic chemical vapor deposition (MOCVD), were used to produce quantum dots by strain confinement. Lateral confinement of carriers in a GaAs AlGaAs quantum well located near the surface was obtained from the inhomogeneous strain produced by the InP islands. The evolution of the InP islands on Al 0.3Ga 0.7As surfaces with increasing InP coverage at different growth temperatures and substrate orientations was studied using atomic force microscopy. Under certain growth conditions, a fairly uniform distribution of coherent InP islands was obtained which had an average apparent diameter of 140 nm with a standard deviation of 12.2 nm and height of 19.5 ± 1.1 nm. Lateral confinement depths up to 100 meV were obtained when using the islands as stressors. Photoluminescence from ensembles of the strain-induced dots exhibit peaks, narrow line widths (16 meV) and high efficiency up to room temperature.

Charge-separated state in strain-induced quantum dots

We have measured the time-resolved photoluminescence of strain-induced quantum dots. We show that a long-lived intermediate state is involved in the excitation transfer from the interstitial quantum well to the dot. This intermediate state has the properties expected of the charge separated state predicted by theory.

Red luminescence from strain-induced GaInP quantum dots

The strain of self-organized InP islands is used to induced quantum dots in near-surface GaInP/AlGaInP quantum wells. To obtain quantum dot luminescence in a widely tunable wavelength range of 630–700 nm, the composition and thickness of the GaInP quantum well is varied. The effect of different cap layer materials, i.e., GaAs, AlGaAs, GaInP, and AlGaInP on the InP island formation and quantum dot luminescence properties is investigated. The luminescence intensity ratio of the quantum dot peak to the quantum well peak is found to be highest when a GaAs cap is used.

Slow relaxation of excited states in strain-induced quantum dots.

We have studied photoluminescence from GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As strain-induced quantum dots in a magnetic field. These dots have high radiative efficiency and long (\ensuremath{\sim}ns) luminescent decay times. At low excitation intensities, corresponding to average carrier densities of less than one electron-hole pair per dot, excited-state (``hot'') luminescence due to slow interstate relaxation is observed. At intermediate intensities, where there are several electron-hole pairs per dot, the hot luminescence disappears, showing that the relaxation rate has increased. However, the excited-state emission reemerges at high excitation when the ground state is saturated. The interstate relaxation rate in the quantum dots under low excitation is at least two orders smaller than that of the host quantum well. The reduced rate is attributed to the discrete density of states in a quantum dot, which inhibits single-phonon emission because the excitons are spatially too large to couple to phonons with the required energy. When there are several electron-hole pairs per dot, carrier-carrier interaction accelerates relaxation. The magnetic field is used to separate the quantum dot states and allows us to probe how their relaxation depends on energy. We find that there is a strong increase in the relaxation rate when the sublevel energy exceeds about 20 meV. \textcopyright{} 1996 The American Physical Society.

Three-Dimensional Carrier Confinement in Strain-Induced Self-Assembled Quantum Dots

Recent technological and materials advances in semiconductors have brought about the possibility of producing heterostructures within which carriers are confined to an ultrasmall region of space (a few thousand atoms) by a potential barrier. When the dimensions of the confining potential are smaller than the electron wavelength (a few tens of nanometers), the semiconductor electronic and optical properties are drastically altered. In these so-called quantum structures, carrier energy levels are quantized and their energy depends on the confining-potential dimensions and magnitude.Some of these quantum structures have already found technological applications. For example the quantum-well (QW) semiconductor laser is part of every CD player. It is also widely used as the light source for intercontinental optical communications. The carrier confining potential in this case is provided by two wider bandgap semiconductor layers sandwiching a thin (3–20 nm) smaller bandgap semiconductor film. The carriers have two degrees of freedom within the QW. The QWs are grown by epitaxial deposition on a crystalline substrate. The substrate may or may not be lattice-matched with the epitaxial film. In some instances, a small lattice mismatch may be required to obtain the desired band-gap value for the QW material. These are the so-called pseudomorphically strained QW structures and devices.

Recombination processes in strain-induced InGaAs quantum dots

We have performed time-integrated and time-resolved photoluminescence experiments on high-quality stressor-induced InGaAs/GaAs quantum dots. The optical spectra of these dot structures exhibit large quantization effects together with remarkably low inhomogeneous broadenings of the respective excitonic transitions. Thus a detailed investigation of the relaxation and recombination dynamics within the distinct electronic dot states becomes feasible. We find that the initial relaxation of optically generated carriers down to the lowest dot states is very efficient. This fast thermalization is ascribed to Coulomb scattering between carriers confined in dot states and carriers located in the higher-energetic quantum well states.

Luminescence from excited states in strain-induced InxGa1-xAs quantum dots.

We have fabricated quantum dots by locally straining ${\mathrm{In}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum wells with self-organized growth of nanometer-scale InP stressors on the sample surface. The structure is completed in a single growth run using metalorganic vapor-phase epitaxy. Photoluminescence from the dots is redshifted by up to 105 meV from the quantum-well peak due to the lateral confinement of excitons. Clearly resolved luminescence peaks from three excited states separated by 16--20 meV are observed when the quantum well is placed at the depth of 1--10 nm from the surface of the sample. The observed redshift and peak separation are in agreement with simple calculations using a finite-element method and two-dimensional parabolic potential model. This structure is easily fabricated and offers a great potential for the optical study of relaxation and recombination phenomena.

Strain-induced quantum dots by self-organized stressors

Novel in situ method to produce quantum dots is reported. Three-dimensional confinement of carriers to a GaInAs/GaAs quantum well dots is observed by photoluminescence. The confinement potential is induced by stressors, formed by self-organizing growth of InP nanoscale islands on top barrier GaAs surface. Two transitions arising from the strain-induced quantum dots produced by two types of InP islands are identified. The luminescence from higher electronic states of the quantum dots having a level splitting of 8 meV is also observed.

Growth of GaAs/AlGaAs Quantum Dots Using Self-Organized InP Stressors

AbstractGaAs quantum dots were formed in a near surface quantum well (QW) by producing lateral confinement with self-organized InP stressors grown in situ by metal organic chemical vapor deposition (MOCVD). We report here the influence of growth conditions on InP island formation on AlGaAs/GaAs single QW structures and also the influence of the QW structure on the optical properties of the GaAs quantum dots. We observe strong photoluminscence up to room temperature from the strain-induced quantum dots with energy redshifts of 70 meV below the QW peak.

Optical Study of Carrier Transfer in Strain-induced Quantum Dots

AbstractWe have studied excitation transfer from the host quantum well to strain-confined quantum dots. We find that there is a long-lived intermediate state acting as a reservoir, holding the excitation before it is thermally activated over a barrier to reach the quantum dot. The barrier height increases monotonically with dot size.